Tokyo also called for China to open up more on Monday.
"Holding the Olympics was good in terms of China taking a more democratic path. We believe this is an irreversible path," Nobutaka Machimura, Japan's chief cabinet secretary, said.
"While the reformist, open-door policy is said to be making progress in China, it is not always leaping forward," the spokesman for the government said.
The Olympics should allow Chinese people to "realise how normal, or abnormal, what they are doing is from an international point of view", he added, responding to a question from a reporter about China's treatment of ethnic minorities.
The eight Americans were detained last week for staging a pro-Tibet protest in Beijing and given 10-day sentences, triggering pressure from Clark Randt, the US ambassador, for their release.
|Pro-Tibet activists carried out several protests during the games [EPA]
They were let go before completing their jail terms.
In addition to the eight Americans, a Briton and a Tibetan-German who were also detained last week, were reportedly also deported.
Activists group Students for a Free Tibet welcomed the news of the release but suggested China was motivated mainly by propaganda concerns.
"After two days of negative publicity over its extrajudicial detention of ten Tibet supporters, the Chinese government is seeking to suppress a story that would have cast a shadow over the closing ceremony of these Olympic Games," said Lhadon Tethong, the group's executive director.
Pro-Tibet campaigners carried out at least eight public protests in Beijing in the run-up to the Olympics and during the games, despite tight security.
No rallies were held throughout the games in three parks designated as protest zones after Chinese officials declined to issue permits to 77 applicants, and detained some of them.
The authorities expelled foreign activists within a day or two after the early protests, but then appeared to toughen their approach by announcing punishments of 10 days in detention.